The controlled radiation of plastics results in the cross-linking of polymer chains and, thereby, provides desirable characteristics to products made from the plastics. It is well known in the plastic films industry that radiation treatment strengthens a film. Various types of coatings are also treated with radiation to improve the bonding of the coating to the object that is being coated. In addition, various adhesive materials are exposed to radiation to promote the curing. This is especially important in production line manufacturing, where the fast curing of an adhesive is required so that the article being produced can be expeditiously moved to the next stage in the production sequence.
A particularly useful form of radiation treatment in manufacturing applications is ultraviolet (UV) curing. UV curing is an electrically based technology which uses ultraviolet radiation generated by a high voltage power supply to rapidly convert certain photo-reactive materials from liquids to solids. These ultraviolet-curable materials include, inks, coatings and adhesives. Although these materials are liquids in their uncured state, they contain no volatile organic compounds (VOCs) and, thus, avoid one of the more serious problems encountered when organic compounds (VOCs) and, thus, avoid one of the more serious problems encountered when using solvent-based adhesives, i.e., the safe venting and treatment of the VOCs before releasing them into the atmosphere.
Polymer-based adhesives are used extensively in many industries, such as the automotive industry, the medical industry, and the electronics industry, as the most efficient means of joining two or more elements. The inherent disadvantage of the use of adhesive is the time factor for setting or curing the adhesive. Air dried adhesives can take from several minutes to several hours to cure and, as a result, make it difficult to implement continuous manufacturing processes. In order to solve this problem, certain polymer-based adhesives have been developed, which cure at an accelerated rate when subjected to ultraviolet radiation. Ultraviolet radiation curing is a process which involves polymerization, or cross linking of monomers upon exposure of the monomer to ultraviolet radiation. For monomers that do not polymerize when exposed to ultraviolet radiation, a sensitizer is added which absorbs ultraviolet energy and initiates a polymerization reaction in the monomer. The use of ultraviolet radiation provides significant processing and handling advantages during manufacture by instantly immobilizing the resin. Immobilization of the resin is controlled to provide sufficient gelation to prevent flow out of the part but allow good wetting between layers, thus assuring even resin distribution, reduced void formation and ease of handling of the finished part without resin migration, sagging or dripping. In addition to a substantial saving of time, there is also a considerable saving in plant space, since an ultraviolet curing line is faster and considerably shorter than previous systems which utilized other means for accelerating the curing of adhesives, such as ovens. Another advantage of ultraviolet cured adhesives over solvent-based adhesives is that the radiation treatment does not discharge volatile organic compounds into the atmosphere.
While UV curing addresses certain industrial needs, there are safety concerns involved with the use of ultraviolet radiation curing systems. Serious bums to the skin and eyes can be caused by the high intensity of the ultraviolet radiation if adequate shielding in not provided. This problem is complicated by the fact that ultraviolet bums are not felt for several hours, so that serious injuries can occur without the person realizing that the injury is occurring. In addition, a considerable amount of infrared energy is produced by the ultraviolet emitting device which can be harmful to personnel and can damage the products which are treated by the ultraviolet radiation. Although prior art devices provide shields to protect individuals from ultraviolet radiation, these shields are not always effective because manufacturing requirements necessitate that the equipment be designed to allow easy and continuous access to the radiation zone. As a result, many of the shields do not always totally enclose the radiation zone and a certain amount of radiation escapes the system. In other cases, the shields are not adjustable and do not allow the equipment to be used for a variety of different applications.